The present invention relates generally to semiconductor based sensors and specifically to semiconductor on insulator based sensors. A diaphragm type silicon based pressure sensor will be used as an example to describe a problem with sensors in the past. Such a pressure sensor will typically include piezoresistors positioned to sense strain associated with pressure and arranged in a Wheatstone bridge to which a direct current voltage is applied. The output voltage of the bridge is representative of the pressure that is being sensed. When no pressure is sensed the output of the bridge should be zero or null. Slight differences in the bridge resistors or other causes will typically produce some initial offset from null upon power up of the bridge. A power up drift (PUD) phenomena has been observed in silicon based sensors that is not explained by a small thermal rise that may occur after power is applied to the sensor.
The PUD phenomena is apparently a result of charges present in a silicon chip or on the surface of a silicon chip, which have one preferred configuration with power off and a second preferred configuration with power on. That is, these charges move in response to the application of voltage to the silicon chip. As the charges move they apparently affect the characteristics of the circuit elements on the chip. The charges may reside in any of a number of locations in the integrated circuit. They may be in the silicon, in insulating layers on or under the silicon, at the interfaces between two of these layers, or at the surface of the silicon chip. The defects may be charge defects such as dangling bonds, or may be charged impurity ions. This PUD phenomena is typically of little consequence for digital circuit as the change in charge location usually results in circuit changes which are much smaller than the rail voltages used. Sensors, including pressure sensors, are often designed with a bridge configuration to minimize this, and other performance limitations. In a bridge configuration, the change of any one element resulting from the redistribution of charges on power up is not significant as long as its balancing element undergoes the same change. Therefore great care is usually taken in the design of a sensor to insure that the individual elements of the bridge are as identical as possible. The power-up drift of the bridge output "resets" itself after the power is removed to the value that existed before power was applied. Also, the stabilization time of the power-on condition is characteristically longer than the "reset" time of the power-off condition.
Silicon on insulator (SOI) based sensors offer performance advantages over bulk silicon based sensors. For example, SOI sensors allow the use of higher impedance piezoresistors which reduce the sensor power requirements. The higher impedance can be used because in SOI sensors the piezoresistors are isolated from the silicon substrate by an insulative layer. This is in contrast to sensors formed in bulk silicon where leakage currents exist and lower impedance piezoresistors must be used. The low power requirement allows sensors to be used in new applications. Also, as ambient temperature increases, the leakage problems associated with sensors formed in bulk silicon also increase. However, SOI based sensors have resistors that are isolated from the bulk silicon and these sensors perform well at elevated temperatures.
Where the sensor application is such that power is continuously applied to the sensor or where reduced measurement accuracy is acceptable, the PUD may not be a problem. However, numerous applications for sensors require high accuracy and many applications may dictate that the bridge does not have power continuously applied. For example, the bridge may not be used continuously, but it may be desirable because of power supply constraints or other considerations to take readings a short time after the power is applied to the bridge. Power supply constraints may exist, for example, due to techniques that utilize a number of sensors but only apply power to a particular sensor when a reading is to be obtained from that sensor. This technique may be used with a hardwired multiplexing approach. This technique could also be used where radio frequency (RF) signals are used to obtain sensor readings and provide a signal to the sensor so that power is only applied to the sensor at that time. In addition, in certain power limited applications the power to the sensor may result from conversion of RF signals to a direct current voltage to power the sensor. Thus there is a need for a measurement system that uses a sensor formed on a silicon-on-insulator structure and accommodates the PUD phenomena.